1. Field of the Invention
The present invention relates to a mobile radio apparatus which is suitable for use in radio communications performed by digital automobile telephones, portable telephones and so on.
2. Description of the Related Art
The code division multiple access (CDMA) scheme is one of the multiple access techniques which allows a plurality of stations to simultaneously communicate in the same frequency band in radio communications. While other techniques are also known, e.g., the frequency division multiple access (FDMA) scheme and time division multiple access (TDMA) scheme, the CDMA scheme is advantageous over these techniques in that the frequency band is highly efficiently utilized and more users may be accommodated.
The CDMA scheme performs the multiple access by spread spectrum communications which spread the spectrum of information signals to a sufficiently wide band compared with an original information bandwidth for transmission. The direct sequence method is a method which multiplexes a spreading code on an information signal as it is. The frequency hopping method is a method which discretely switches the frequency of a carrier signal modulated by information within a given frequency band. The hybrid CDMA scheme is a type of CDMA scheme which is a combination of the direct sequence method and the frequency hopping method, as described in an article "Coherent Hybrid DS-FFH.CDMA, Basic Performance in Mobile Radio Environment" by Shigeru Tomisato, Kazuhiko Hukawa, and Hiroshi Suzuki (Technical Report of IEICE, RCS92-109, pp. 61-66, January 1993).
A CDMA scheme employing the direct spreading method implies a near-far problem which may occur when a desired transmission station is located far from a receiver and an unwanted transmission station (interfering station) is located near the receiver, so that a received signal from the unwanted transmission station has a received power larger than a received signal from the desired transmission station, whereby the processing gain (spreading gain) is insufficient to suppress the cross-correlation between spreading codes, resulting in disabled communications. For this reason, control of the transmission power in accordance with the state of each transmission channel is essential in the reverse link from a mobile station to a base station for a cellular system employing the direct spreading CDMA scheme.
Also, as a measure to combat fading, which is the cause of a deteriorization in link quality in the land mobile communications, a method has been proposed for compensating for changes in the instantaneous value of the received power by controlling the transmission power.
JP-A-4-502841 shows an example of a transmission power control configuration for a cellular system employing a direct spreading CDMA scheme. FIGS. 1A, 1B illustrate the configuration described in JP-A4-502841. In FIG. 1A, a base station generally denoted by 41 includes data to be transmitted 42; a baseband processing circuit 43; a modulator 44; an adder 45; an antenna 46; an analog receiver 47; a digital receiver 48; a received level detector circuit 49; and received data 50. In FIG. 1B, a mobile station generally denoted by 51 includes an antenna 52; an analog receiver 53; a digital receiver 54; a baseband processing circuit 55; received data 56; data to be transmitted 57; a modulator 58; a control processor 59; and a transmission level control circuit 60.
In the above-mentioned configuration, a forward link from the base station 41 to the mobile station 51 uses a different frequency band from that of a reverse link from the mobile station 51 to the base station 41. This method is called a frequency division duplex (FDD) scheme which is advantageous in that a transmitter and a receiver can be operated simultaneously, the transmitter does not interfere with the receiver, and so on.
Since the FDD scheme uses different frequency bands for the reverse link and the forward link, the reverse link and the forward link exhibit different changes in instantaneous value of a received electric field level due to fading, although they present substantially the same slow changes in central value of the received electric field level. Therefore, the transmission power control method first detects the total power level received at the mobile station 51 in the digital receiver 54, and uses the detected total received power level to control the transmission power level by the transmission level control circuit 60, thereby compensating for changes in the central value of an electric field level to be received by the base station 41. This method is referred to as a method of controlling transmission power using a loop. Further, since an open loop alone cannot provide compensation for changes in the instantaneous value of the received electric field level, the base station 41 has the received level detector circuit 49 detect the power level of the received signal from the mobile station 51 after the received signal has been demultiplexed. The detected power level permits the base station 41 to know a link state in the frequency band of the reverse link. Then, the base station 41 transmits reverse link state information to the mobile station 51 through the forward link. The mobile station 51 controls the transmission power using the reverse link state information to compensate for changes in the instantaneous value of the received electric field level due to fading. This method is referred to as a method of controlling transmission power using a closed loop.
As described above, the CDMA scheme using FDD realizes transmission power control by a combination of an open loop and a closed loop.
An article "Power Control in Packets Switched Time Division Duplex Sequence Spread Spectrum Communications" by R. Esmailzadeh, M. Nakagawa, and A. Kajiwara (proc. of VTC' 92, pp. 989-992, 1992) discloses a method of controlling transmission power in a CDMA/TDD scheme. TDD (Time Division Duplex) means that the same band is used for both transmission and reception, and is also referred to as a ping-pong scheme. More specifically, the TDD is a scheme which time divides the same radio frequency band for transmission and reception to achieve bi-directional communications through the same frequency band. FIG. 2 graphically represents a concept of the TDD scheme. It can be seen that at time T.sub.1 the base station starts transmission while the mobile station starts reception, and at time T.sub.2 the mobile station is switched to transmission while the base station to reception. These operations are repeated to realize bi-directional communications through a single frequency band.
The transmission power control in the CDMA/TDD scheme is performed only with an open loop. A control system for the CDMA/TDD scheme is described below with reference to FIGS. 3A, 3B. In FIG. 3A, a base station generally denoted by 61 includes data to be transmitted 62; a modulator 63; a spreader 64; a switch 65; an antenna 66; a correlator 67; a demodulator 68; and received data 69. In FIG. 3B, a mobile station generally denoted by 71 includes an antenna 72; a switch 73; a correlator 74; a demodulator 75; received data 76; data to be transmitted 77; a modulator 78; a received level detector circuit 79; a transmission level control circuit 80; and a spreader 81.
In the above-mentioned configuration, at a timing such as time T1 in FIG. 2, at which the base station 61 starts transmission while the mobile station 71 starts reception, the switch 65 in the base station 61 connects the spreader 64 to the antenna 66, while the switch 73 in the mobile station 71 connects the correlator 74 to the antenna 72. In the base station 61 thus connected, the data to be transmitted 62 is modulated in the modulator 63, spread in the spreader 64, and transmitted from the antenna 66. In the mobile station 71, in turn, the signal transmitted from the base station 61 through a transmission channel is received by the antenna 72, subjected to detection of a correlation in the correlator 74, and demodulated to received data in the demodulator 75. The output of the correlator 74 is inputted to the received level detector circuit 79, which detects therefrom the received power on the communication channel directed to the mobile station 71. Next, at time T.sub.2 in FIG. 2, at which the mobile station is switched 71 is switched to transmission while the base station to reception, the switch 73 in the mobile station 71 connects the spreader 81 to the antenna 71, while the switch 65 in the base station 61 connects the correlator 67 to the antenna 66. In the mobile station 71, data to be transmitted 77 is modulated in the modulator 75, and transmission power is determined in the transmission level control circuit 80 using the output of the received level detector circuit 79 generated at time T.sub.1 such that the power level to be received at the base station 61 remains constant irrespective of fading and so on. A signal to be transmitted, after being outputted from the transmission level control circuit 80, is spread in the spreader 81, and transmitted from the antenna 72. The base station 61, in turn, receives multiplexed signals from a plurality of mobile stations 71 through transmission channels by the antenna 66, separates the signal from the particular mobile station 71 by the correlation detection, and demodulates the separated signal in the demodulator 68 to derive received data 69.
As described above, since the CDMA/TDD scheme uses the same frequency band for the reverse link and the forward link, these links present the same change in distance of a radio link and the same change in instantaneous value of the received electric field level due to fading. Therefore, the open loop transmission power control is only required to compensate for a slow change with respect to the period of TDD.
However, in the transmission power control for the CDMA/IDD scheme, since different frequency bands are used for the reverse link and the forward link, although these links present substantially the same slow changes in central value of the received electric field level, changes in the instantaneous value of the received electric field level due to fading are not the same, so that a closed loop configuration is required for the transmission power control. For such a closed loop control, the base station must be provided with a means for detecting the received levels of all associated mobile stations in order to reveal the link state for the reverse link and a means for informing the associated mobile stations of the state information, thus resulting in a complicated system. Also, for the mobile stations to know the link state, a delay occurs due to the processing for detecting the received levels of mobile stations in the base station, the propagation time required to send the state information to the mobile stations, and so on. It is therefore impossible to compensate for a change faster than the delay time.
Further, in the transmission power control for the CDMA/TDD scheme, since the same frequency band is used for the reverse link and the forward link, the mobile station can know the link state only with a received signal from the base station, and therefore a closed loop configuration is only required for the transmission power control. However, during a state in which the mobile station is dedicated to transmission and the base station to reception, the mobile station cannot know the link state during this state since it is the transmitting party. Even if a sudden change occurs in received power due to shadowing of a building or the like during this state, the mobile station cannot follow such a change. Particularly, if the received power suddenly becomes extremely large, this change will adversely affect the entire system. Also, since each mobile station detects the received power level of a signal on a communication channel directed thereto after the correlation detection, an auto-correlation value and a cross- correlation value differ from one mobile station to another due to different spreading codes and different communication channel data between the respective mobile stations. As a result, the respective mobile stations present different accuracies in the detection of the received power level. Further, when the base station also controls the transmission power, the base station cannot exactly know the link states at the respective mobile stations since the transmission level of the base station changes.